In the following description of the invention, problems and solutions will be described in relation to broadband signaling in the presence of one or more narrowband interferers. To simplify and clarify the description, WCDMA (Wideband Code Division Multiple Access) and HSPA (High Speed Packet Access) will be used as an example broadband signaling and GSM (Global System for Mobile communication) will be used as an example narrowband signaling. However, embodiments of the invention are applicable also to other cases of broadband signaling in the presence of one or more narrowband interferers. Examples of broadband signaling are WCDMA, UMTS LTE (Universal Mobile Telecommunications System Long Term Evolution) and UMTS LTE Advanced. Examples of narrowband interferers are GSM and EDGE (Enhanced Data rates for GSM Evolution).
High data rate cellular systems, such as UMTS (Universal Mobile Telecommunications System) or UMTS LTE (UMTS Long Term Evolution) employing techniques such as WCDMA and HSPA, may be introduced in spectrum, which is also used by GSM systems.
WCDMA/HSPA systems are designed to use a bandwidth of 5 MHz or a multiple of 5 MHz. Operators running GSM-networks may not have at their disposal such bandwidths of free spectrum for operating WCDMA/HSPA. Hence, a GSM-operator expanding its business to WCDMA/HSPA-networks may have to either completely abandon its GSM operation, which has obvious disadvantages from a business perspective, or try to apply WCDMA/HSPA operation in the same or overlapping spectrum as GSM operation.
In a migration scenario from GSM to WCDMA/HSPA, an operator may want to allocate as small bandwidth as possible to WCDMA/HSPA. One reason might be that the operator does not want to reduce the capacity of the GSM operation too much. The theoretical maximum bandwidth of WCDMA/HSPA may be derived through studying the pulse shaping for WCDMA/HSPA. The theoretical maximum bandwidth is 4.7 MHz (with reference made to 3GPP specification TS25.101, 3.84 *1.22=4.7) if the 5 MHz version of WCDMA/HSPA is considered. Since the theoretical maximum bandwidth is less than 5 MHz, it might occur to operators to try to squeeze in both the WCDMA/HSPA bandwidth and the GSM bandwidth in, for example, a 5 MHz spectrum.
Furthermore, if relying on that the majority of the information in WCDMA/HSPA is within 3.84 MHz bandwidth (see 3GPP specification TS25.101), an operator might try to put a GSM carrier as close as, for example, 2.2 MHz from the WCDMA/HSPA carrier to optimize the bandwidth utilization. (Placing the GSM carrier 2.4 MHz from the WCDMA/HSPA carrier would work quite easily and the GSM signal would not interfere with the WCDMA/HSPA signal. Since a carrier spacing of 0.2 MHz is applicable in GSM, 2.2 MHz spacing between GSM and WCDMA carriers would be the next obvious alternative to exploit.)
FIG. 1 illustrates an example scenario where the GSM carrier 101 is placed Δf MHz from the WCDMA/HSPA carrier fc 102. As can be seen the GSM signal 103 is within the WCDMA/HSPA bandwidth and will propagate into the WCDMA/HSPA receiver chain, since it is within the bandwidth of a possible WCDMA/HSPA receive (RX) filter passband 105. The GSM signal 103 thus acts as an interferer for the WCDMA/HSPA signaling 104. Hence, there is a risk that the GSM signal (or GSM interferer) 103 blocks or impairs part of the WCDMA/HSPA information signal, which may degrade the reception performance significantly for WCDMA/HSPA.
A straightforward approach to solving this problem is illustrated in FIG. 2 and comprises applying a narrower RX filter 205 to filter out (at least part of) the GSM interferer 103. Such an RX filter would be narrower than the standardized WCDMA/HSPA signal bandwidth. A disadvantage with such a solution is that some of the information in the WCDMA/HSPA signal is lost. This will be the case regardless of the strength of the GSM interferer or even the existence of a GSM interferer. A throughput degradation and capacity loss is thus experienced in the WCDMA/HSPA system.
U.S. Pat. No. 7,221,958 B2 discloses a filtering technique which positions one or more filter nulls substantially at points of narrowband interference in a relatively wideband received signal. The technique may be used for removing adjacent channel interference in a received WCDMA signal caused by GSM radio transmissions. Such operations, however, alter the channel as it is seen by the receiver's signal processing chain subsequent to the filtering. Thus, the performance and accuracy of the subsequent processing is not optimal if such filter nulls are applied.
Hence, there is a need for improved methods, devices and systems for processing a signal in the presence of narrowband interference.